In continuous ink jet printing, streams are discharged from an orifice or array of orifices to form droplet streams. To regulate the streams' breakup into uniformly sized and spaced drops, series of energy pulses of predetermined frequency are applied to the ink stream. One preferred mode for applying the pulse series that stimulate uniform droplet streams is by vibration, e.g., of the orifice plate, a resonator housing or the ink volume behind the orifices. When the issuing ink streams (called filaments) break up properly into droplet streams, the filament tip separates into a droplet within a predetermined drop charge region, e.g., adjacent a charge electrode. The charge electrode is energized with a charge voltage, or is not so energized, in accord with an information signal; and because the ink is conductive and grounded, a charge is correspondingly induced, or not induced, on the drop then forming at the drop charge region. Ink droplets are thereafter passed to the print zone, or are caught, in accord with their charged or non-charged conditions.
It will be appreciated from the foregoing that it is important, for good printing operations, to assure the ink filaments break up into drops within the nominal charging region (i.e. within a given range of locations along the drop path that is acceptably close to the charge electrode). Drop breakup before or beyond this nominal charging region can result in improper drop charging.
Another important factor for obtaining reliable printing is that the stimulating energy be applied in a manner that avoids formation of small satellite drops during the occurrences of filament break off. This is because it is difficult to control drop charging and deflection in the presence of such satellite drops. It has been recognized, e.g, see U.S. Pat. No. 4,631,549, that as the amplitude of drop-stimulating energy increases, the drop break off conditions change from an "underdrive" stimulation region where satellites (both of the merging and infinite type) are formed to a satellite-free region where no satellites are formed to an "overdrive" condition where satellites are again formed. There is a direct relation between filament length (i.e. the length from orifice to drop break off point) and the domains of satellite and non-satellite drop formation, and it is desired to operate at a stimulation amplitude that achieves both the proper break off point location and the desired non-satellite domain of drop formation. However, in addition to dependence on stimulation energy amplitudes, the domains where satellite and non-satellite drop formation occur depend on other system parameters (e.g., temperature, ink viscosity and ink pressure). Those other parameters can vary gradually over periods of time, and prior art techniques have been developed to periodically check and adjust the stimulation amplitude, in view of such variations, to assure optimum drop charging and deflection.
In the procedure taught by U.S. Pat. No. 4,631,549, the operating stimulation amplitude is obtained by detecting the stimulation amplitude at the infinite satellite condition (with an electrometer) and selecting the operating amplitude to be a value that is a predetermined multiple of the detected infinite satellite stimulation amplitude. However, the infinite satellite detection and adjustment approach is sometimes hard to effect, e.g., when satellites are smaller than normal due to lower ink pressures. Also, the optimum operating point amplitude sometimes varies relative to the infinite satellite amplitude from a fixed predetermined multiple value, depending, e.g., on temperature, pressure, orifice size and ink properties.
U.S. Pat. No. 4,638,325 describes a procedure for assessing ink jet filament lengths in order to select an appropriate operating stimulation amplitude. This procedure employs a narrow test charge electrode located at the information charge electrode position (or directly opposite that position across the drop path). As stimulation amplitude is increased from a lower amplitude range to higher amplitudes, the filament length gradually shortens. The small charge imparted to ink drops by the test charge electrode gradually increases as the filament break off point moves upwardly toward the test charge electrode, and then begins to decrease as the stimulation passes into the overdrive region and the filament again begins to lengthen. The charges imparted to ink drops are measured at the various stimulation amplitude stages by an electrometer and the stage of maximum imparted charge is identified as the minimum filament length, and thus the overdrive inception point. A desired operating amplitude for stimulation is then selected.
The '325 patent procedure is very useful, but it requires that the test charge electrode be precisely located vis-a-vis the minimum filament length of the printing system. Otherwise, a filament can continue to shorten, above the test charge electrode and be mistaken for a filament increasing in length below the test charge electrode. Also, locating a test charge electrode at or opposite the information drop charge site presents construction difficulties.